Myotonic dystrophy (DM), the most common form of muscular dystrophy in adults, can be caused by a mutation on chromosome 19 (DM1) or 3 (DM2/PROMM). DM1 is caused by a CTG expansion in the 3' untranslated region of the dystrophia myotonica-protein kinase gene (DMPK) and the promoter region of the homeodomain gene SIX5. Although the DM1 mutation was isolated in 1992, for many years it has been difficult to understand how this mutation, which does not alter the protein-coding portion of a gene, causes this dominantly-inherited multisystemic disorder. Suggested mechanisms have included: DMPK haploinsufficiency; reduced expression of regional genes including the homeodomain gene SIX5; and pathogenic effects of the CUG expansion in RNA. Mouse models have suggested that each of these mechanisms contributes to DM1 pathogenesis and that DM1 may be a regional gene disorder. To clarify the pathogenic mechanism of DM, we have studied a second form of myotonic dystrophy that causes a strikingly similar constellation of clinical features in humans including myotonia, myopathy, iridescent cataracts, cardiac arrhythmias, and the specific set of serological changes characteristic of DM. We mapped the DM2 locus to chromosome 3q21 (Nature Genetics 19:196-198, 1998) and recently reported that DM2 is caused by a repeat expansion in intron 1 of the zinc finger protein nine (ZNF9) gene (Science, 293:864-867). Clinical and molecular parallels between DM1 and DM2 indicate that CUG and CCUG expansions expressed at the RNA but not the protein level can themselves be pathogenic and cause the multisystemic features common to both diseases. We propose generating transgenic mice and cell culture models to test the effects of CCUG repeat-containing transcripts and to better understand the molecular mechanisms involved in the disease process. Our specific aims are: (1) To generate a reversible murine model to test the pathogenicity and characterize the downstream effects of transcripts containing the CCUG expansion expressed in skeletal muscle. (2) To generate and characterize a BAC transgenic mouse model that replicates the multisystemic features of myotonic dystrophy type 2. (3) To develop a cell culture model to evaluate the molecular effects of CCUG length on the formation of RNA foci, cellular differentiation and downstream molecular changes in alternative splicing.